In order that an orderer may pre-assign colors for a certain portion of printed matter, a small sheet of paper for assigning colors called a color specimen has sometimes been attached to a layout. For example, in the case where a background of a picture is desired to be evenly painted with a specific color, a small sheet of paper painted with the color of said background is attached as a color specimen and is sent to a printing factory.
The printing factory employs a four-color printing system using color separations of C, M, Y and Bk, which is the commonest multicolor printing system, and decides what percent of half-tone dot percents of color separations of C, M, Y and Bk should be used to print the assigned portion with the same color as that of the color specimen before preparation of color separations under such conditions and printing.
In the past, the above-described work includes: preparing a color chart which is an assembly of a multiplicity of printed matters, said color chart being printed with half-tone dot percents of color separations of C, M, Y, and Bk varied at intervals of about 10%; visually comparing the color specimen with colors on the color chart by an operator to select a color most approximate to the color specimen from the color chart; utilizing the fact that half-tone dot percents of color separations necessary for printing colors in the color chart are described in the color chart to fine half-tone dot percents of color separations necessary for printing said selected color; preparing color separations having said half-tone dot percents; and reproducing colors assigned by the color specimen.
However, this method has many disadvantages, because the work requires manual operations, the kinds of colors in the color chart are very great in number, comparison work for comparing colors in the color chart with the color specimen takes much time, and the colors selected from the color chart differ depending on operators, and thus, irregularities in colors to be reproduced tend to occur.
On the other hand, a reflection densitometer is now commercially available which can measure density of an article to be measured to figure out a half-tone dot percent. According to this densitometer, the measured value of density is processed by a computer housed in the densitometer to calculate a half-tone dot percent.
In this densitometer, a Yule-Nielsen's equation is used to calculate a half-tone dot percent. However, the scope of application of the Yule-Nielsen's equation has a limitation.
That is, this equation is based on the assumption that the relation with a standard density of half-tone dot percent 100% is utilized, and therefore, only the color having said standard density can be applied thereto and, in addition, the equation can be normally used only when an article to be measured has a mono-color. Moreover, this equation is realized by the fact that dots are reproduced ideally without dot gains or the like and that the coefficients may vary due to factors of screen rulings, density, and quality of paper. Therefore, in actual use, even with a mono-color it is not possible to accurately obtain half-tone dot percents in a wide range such as from a light area to a shadow area. Further, it is impossible to calculate half-tone dot percents for more than a secondary color.
Accordingly, such a densitometer cannot be used for work for obtaining half-tone dot percents of color separations of C, M, Y and Bk which can reproduce colors assigned by the color specimen.
Also, with a conventional densitometer as described, it is not possible to accurately obtain half-tone dot percents of color separations for reproducing colors on the color specimen for the following additional reasons.
That is, printing ink of C, M and Y are not ideal, and an M component and a Y component are included in C ink, a C component and a Y component are included in M ink, and a C component and a M component are included in Y ink.
For example, suppose that an image of half-tone dot percent 100% is printed with only C ink of certain ink makers. When densities of C, M and Y of this printed matter are obtained through R (red) filter, G (green) filter and B (blue) filter, respectively, they are 1.53, 0.52 and 0.17, respectively, and despite the fact that printing is made only with C ink, and M component and a Y component are included. When the Yule-Nielsen's equation is used to obtain a half-tone dot percent, the half-tone dot percent of each color separation will have the value thus corresponding to the aforesaid density value, failing to obtain a proper half-tone dot percent. Such a phenomenon becomes more complicated in case of a printed matter in which ink of plural colors are printed.
It can be understood even from the above-described reason that the densitometer using the Yule-Nielsen's equation cannot be applied to the afore-mentioned work.
Some of the colors assigned by the color specimen are not present in the color chart. It is well known that bright colors among the secondary colors, for example, such as green, orange, purple and the like are difficult to reproduce in color-printing with ordinary process ink.
This largely results from the fact that chromaticity of process ink is relatively strained as compared with chromaticity of the theoretical three primary colors for the reasons of economy, printability, light-resistance, water resistance and the like, and therefore, turbidity arises. When such a color is compared with ones on the color chart to compare it with a color which seems to be closest thereto, then, the color would be judged to be a color which is perceived as being difficult to reproduce. However, it is not possible to quantitatively grasp to what extent the color cannot be reproduced.
Accordingly, in actual work, a color which appears closest is selected out of the color chart, and half-tone dot percents of color separations are corrected so as to make that color approximate to that of the color specimen by retouching work before printing. However, it is often encountered that when the results of printing are checked, the reproduced color is greatly different from the color of the color specimen and the half-tone dot percents have to be changed. Considerable cost of material and loss of time result for such procedure. This problem is impossible to solve as long as one relies upon work by hand and upon the aforesaid densitometer.